1. Field of the Invention
The present invention relates to a type of electromagnetic resonator, which may be called "twin strip resonator" as well as to the high frequency filters constructed from these resonators.
2. Description of the Prior Art
In the high frequency range called UHF (in practice from 300 MHz to 3 GHz), the resonators and the filters constructed from these elements are often formed from line sections. They may be air coaxial lines or dielectric charged coaxial lines such as mentioned in the article: "Bandpass filter with dielectric materials used for broadcasting channel filter" by K. WAKINO and Y. KONISHI published in the I.E.E.E. review Transactions on Broadcasting, vol BC-26, No. 1, March 1980. It is also known to manufacture resonators and filters from microstrip lines as mentioned in the article: "750 MHz microstrip bandpass filter on barium tetratitanate substrate" by G. OHM and G. SCHMOLLER published in the review Electronics Letters, vol 18, No. 15 of July 22, 1982.
The technique of coaxial lines allows the manufacture of independent resonators whose natural frequencies may be adjusted before assembly thereof to form filters. This assembly may be achieved in the case of a passband filter by placing the different resonators end to end, the couplings between two consecutive line sections being determined by the distances which separate their oppositely located faces. However, to obtain interesting Q factors (greater than 500) line sections are required having a fairly large cross section. Typically, a silver metallized resonator with a diameter of 20 mm may have a Q factor greater than 1000 for a frequency of 1 GHz. Moreover, the coupling of quarter-wave resonators remains delicate and the construction itself of the coaxial structure is fairly complex because of the different operations for machining and metalizing circular section elements.
Resonators may be designed according to the technique of microstrip lines. They are generally formed from a relatively wide dielectric substrate one face of which is entirely metalized and the other face of which receives a metal conductor in the form of a thin ribbon. This technique has two disadvantages. On the one hand, the natural Q factors of the resonators are always low (less than 500) and, consequently, the performances of filters formed from these resonators are always modest (high insertion losses, greater than 3 dB at about 1 GHz). On the other hand, once the filter has been constructed, by depositing strips on the same substrate, it is practically impossible to adjust the natural frequencies of the resonators as well as their mutual coupling. This puts an obstacle in the way of the industrial production of filters comprising a high number of poles because of the inevitable dispersions of the characteristics: in particular, of the dielectric constant of the substrate.